The invention relates to a microlithography objective, a projection exposure apparatus containing the objective, and a method of manufacturing an integrated circuit using the same.
Using a lithography system operating with wavelengths below 193 nm for imaging structures of below 130 nm resolution has been proposed. In fact, such lithography systems have been suggested for the extreme ultraviolet (EUV) range with wavelengths of xcex=11 nm or xcex=13 nm producing structures of below 100 nm. The resolution of a lithographic system is described by the following equation:
RES=kixc2x7xcex/NA
where k1 is a specific parameter of the lithographic process, xcex is the wavelength of the incident light, and NA is the image-side numerical aperture of the system. For example, if one assumes a numerical aperture of 0.2, then the imaging of 50 nm structures with 13 nm radiation requires a process with k1=0.77. With k1=0.64, the imaging of 35 nm structures is possible with 11 nm radiation.
For imaging systems in the EUV region, substantially reflective systems with multilayer coatings are available as optical components. Preferably multiple layers of Mo/Be are used as multilayer coating systems for systems operating at xcex=11 nm, whereas Mo/Si systems are used for xcex=13 mm. With the reflectivity of the multilayer coatings approximating 70%, it is desirable to use as few optical components as possible in applications such as EUV projection objective microlithography to achieve sufficient light intensity. Specifically, to achieve high light intensity and to allow for the correction of imaging errors, systems with six mirrors and a numerical aperture (NA)=0.20 have been used.
The six-mirror systems for microlithography have become known from the publications U.S. Pat. No. 5,686,728, EP 779,528 and U.S. Pat. No. 5,815,310. The projection lithography system according to U.S. Pat. No. 5,686,728 has a projection objective with six mirrors, where each of the reflective mirror surfaces has an aspherical form. The mirrors are arranged along a common optical axis in such a way that an obscuration-free light path is achieved. Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high asphericity of approximately xc2x150 xcexcm as well as very large angles of incidence of approximately 38xc2x0. Even after reducing the aperture to NA=0.2, an asphericity of 25 xcexcm from peak to peak remains, with little reduction in the angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region according to the present state of the art because of the higher requirements on surface quality and reflectivity of the mirrors in these latter systems.
Another disadvantage of the objectives disclosed in U.S. Pat. No. 5,686,728, which precludes their use with wavelengths below 100 nm such as the 11 nm and 13 nm wavelengths desirable for EUV microlithography, is the short distance between the wafer and the mirror lying closest to the wafer. This short distance allows only very thin mirrors to be used in the U.S. Pat. No. 5,686,728 apparatus. Because of the extreme stresses on the coatings of the multilayer systems suitable for the 11 nm and 13 nm wavelengths in question, such thin mirrors are very unstable.
A projection objective with six mirrors for use in EW lithography, even at wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage that at least two of the six mirrors have very high asphericities of 26 and 18.5 xcexcm. Unfortunately, in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer is so small that either instabilities occur or a negative mechanical free working distance is obtained.
Thus, it is desirable to provide a projection objective for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art described above.
According to an aspect of the invention, the shortcomings of the prior art are overcome by using a projection objective with six mirrors where the mirror nearest to a wafer to be illuminated is arranged in such a way that an image-side numerical aperture NAxe2x89xa70.15. Furthermore, the mirror nearest to the wafer is arranged in such a way that the image-side optical free working distance corresponds at least to the used diameter of the mirror next to the wafer; the image-side optical free working distance is at least the sum of one-third of the used diameter of this nearest mirror and a length between 20 and 30 mm; or the image-side optical free working distance is at least 50 mm. In a preferred embodiment, the image-side optical free working distance is 60 mm.
According to another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture, NA, is greater than 0.15 and the arc-shaped field width, W, at the wafer lies in the range 1.0 mmxe2x89xa6W, and the peak-to-valley deviation, A, of the aspheres is limited with respect to the best fitting sphere in the used range on all mirrors by:
Axe2x89xa619 xcexcm-102 xcexcm(0.25xe2x88x92NA)xe2x88x920.7 xcexcm/mm(2 mmxe2x88x92W).
In a preferred embodiment, the peak-to-valley distance A of the aspheres is limited on all mirrors by:
Axe2x89xa612 xcexcm-64 xcexcm(0.25xe2x88x92NA)xe2x88x920.3 xcexcm/mm(2 mmxe2x88x92W).
According to yet another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture NAxe2x89xa70.15 and an image-side width of the arc-shaped field Wxe2x89xa71 mm, and the angle of incidence AOI relative to the surface normal is limited for all beams on all mirrors by:
AOIxe2x89xa623xc2x0xe2x88x9235xc2x0(0.25xe2x88x92NA)xe2x88x920.2xc2x0/mm (2 mmxe2x88x92W).
Preferably, an embodiment of the invention would encompass all three of the above aspects, e.g., an embodiment in which the free optical working distance would be more than 50 mm at NA=0.20 and the peak-to-valley deviation of the aspheres, as well as the angles of incidence, would lie in the regions defined above.
The asphericities herein refer to the peak-to-valley (PV) deviation, A, of the aspherical surfaces with respect to the best fitting sphere in the used range. The aspherical surfaces are approximated in the examples by using a sphere with center on the figure axis vertex of the mirror and which intersects the asphere in the upper and lower endpoint of the useful range in the meridian section. The data regarding the angles of incidence always refer to the angle between the incident beam and the normal to the surface at the point of incidence. The largest angle of any incident light occurring on any of the mirrors is always given, i.e., the angle of a bundle-limiting beam. The used diameter will be defined here and below as the envelope circle diameter of the used region, which is generally not circular.
Preferably, the wafer-side optical free working distance is 60 mm.
The objective can be used not only in the EUV, but also at other wavelengths, without deviating from the scope of the invention. In any respect, however, to avoid degradation of image quality, especially degradation due to central shading, the mirrors of the projection objectives should be arranged so that the light path is obscuration-free. Furthermore, to provide easy mounting and adjusting of the system, the mirror surfaces should be designed on a surface which shows rotational symmetry to a principal axis (PA). Moreover, to have a compact design with an accessible aperture and to establish an obscuration-free path, the projection objective devices are designed to produce an intermediate image, preferably formed after the fourth mirror. In such systems, it is possible for the aperture stop to lie in the front, low-aperture objective part, with a pupil plane conjugated to the aperture stop imaged in the focal plane of the last mirror. Such a system ensures a telecentric beam path in the image space.
In an embodiment of the invention, it is provided that the freely accessible aperture stop be arranged optically and physically between the second and third mirror. Good accessibility of the aperture stop is ensured when the ratio of the distance between the first and third mirror to the distance between the first and second mirror lies in the range of:
0.5 less than S1S3/S1S2 less than 2.
Furthermore, in order to prevent vignetting of the light running from the third to the fourth mirror, by the aperture stop arranged between the second and third mirror, the ratio of the distance between the second mirror and aperture stop to the distance between the third mirror and the aperture stop lies in the range:
0.5 less than S2 aperture/(S3 aperture) less than 2.
By using such an elongated system, the angular loads in the front part of the projection objective can also be reduced.
An aperture stop which physically lies between the second mirror, S2, and the first mirror, S1, must be formed at least partially as a narrow ring in order to avoid clipping of light moving from S1 to S2. In such a design, there is a danger that undesirable direct light or light reflected on S1 and S2, will pass outside the aperture ring and reach the wafer. However, if the aperture stop is placed optically between the second and third mirror and physically close to the first mirror (which can be easily achieved mechanically), an efficient masking of this undesired light is possible. The aperture stop can be designed both as an opening in the first mirror or lying behind the first mirror.
In another embodiment of the invention, the aperture stop is arranged on or near the second mirror. Arrangement of the aperture on a mirror has the advantage that it is easier to achieve mechanically. Here, in order to ensure an obscuration-free beam path with simultaneously low angles of incidence, the ratio of the distance between the first and third mirrors (S1S3) to the distance between the first and second mirrors (S1S2) lies in the range:
0.3xe2x89xa6S1S3/S1S2xe2x89xa62.0,
while the ratio of the distance between the second and third mirrors (S2S3) to the distance between the third and fourth mirrors (S3S4) lies in the range:
0.7xe2x89xa6S2S3/S3S4xe2x89xa61.4.
In order to be able to make the necessary corrections of the imaging errors in the six-mirror systems, in a preferred embodiment, all six mirrors are designed to be aspherical. However, an alternative embodiment whereby at most five mirrors are aspherical thus simplifying the manufacturing process can be acheived. Then it is possible to design one mirror, preferably the largest mirror, i.e., the quaternary mirror, in the form of a spherical mirror. Moreover, it is preferred that the second to sixth mirror be in a concavexe2x80x94convexxe2x80x94concavexe2x80x94convexxe2x80x94concave sequence.
In order to achieve a resolution of at least 50 nm, the design part of the rms wavefront section of the system should be at most 0.07xcex and preferably 0.03xcex.
Advantageously, in the embodiments of the invention, the objectives are always telecentric on the image-side. In projection systems which are operated with a reflection mask, a telecentric beam path on the object-side is not possible without illumination through a beam splitter which reduces the transmission strongly. One such device is known from JP 95 28 31 16. Therefore, the main beam angle on the reticle is chosen so that vignetting-free illumination is possible. Alternatively, in systems with transmission mask, the projection objective can be telecentric on the object side. In these embodiments, the first mirror is preferably concave. Overall, the telecentericity error on the wafer should not exceed 10 mrad and is typically between 5 mrad and 2 mrad, with 2 mrad being preferred. This ensures that changes of the imaging ratio remain within tolerable limits over the depth of focus.
In any of the embodiments of the invention, the six mirror object could additionally include a field mirror, a reducing three-mirror subsystem, or a two-mirror subsystem.
In addition to the projection objective according to the invention, the invention also makes available a projection exposure apparatus, that includes at least one projection objective device. In a first embodiment, the projection exposure apparatus has a reflection mask, while in an alternative embodiment, it has a transmission mask. Preferably, the projection exposure apparatus includes an illumination device for illuminating an off-axis arc-shaped field and the system is designed as an arc-shaped field scanner. Furthermore, the secant length of the scan slit can be at least 26 mm and the ring width be greater than 0.5 mm, so that homogeneous illumination becomes possible in the apparatus.
The invention will be described below with the aid of the drawings as examples.